Really, all you need for a homebirth is mom and a baby ready to come out! But we had a bounty of supplies ready, mostly relating to my desire to labor and birth in the water for pain relief and to decrease the risk of tearing.
For the birth pool, we bought an inflatable child's fishy pool, from http://www.qualityinflatables.com (the 60" round one is what we chose, it was about $22 shipped.). This pool has strong enough sides that you can sit on them, and the bottom is also inflated for comfort. It is also the perfect diameter to allow the mother to brace her feet on the sides while sitting in it. My midwife also commented that it was a perfect size for her to be able to reach me no matter what position I was in and the sides werent too high so it was comfortable to reach over. The pool stays warm for quite awhile because the air in the sides works as insulation; you can always refill if necessary with more hot water, and put a comforter over top to keep it warm if you arent in it yet. I inflated the pool by mouth, over the course of a few weeks, but I'd get a pump next time! To fill the pool, we bought a "drinking water safe" hose from Target, 25 ft long, and screwed it on to the washing machine hookup which has a hose-type connector already (you can also get an adapter kit for the kitchen sink). We also had another hose that we used to help empty the pool, with the help of a $6 electric drill-powered pump from Home Depot (in the plumbing section, you definitely need a plug-in drill for it so we bought that too -- tried our cordless drill but it wasn't powerful enough). To complete the pool supplies we had 2 plastic tarps ($4) and an old sheet to cover them so they wouldn't be slippery when they got wet.
We set the pool up in the middle of the dining room and it took about 10 minutes to empty the hot water tank and get the pool about 1/2 full and a comfortable temperature (just hook the hose to the hot water and fill it with as much and as hot water as you can get, then after the tank is empty fill it with the cold that comes out until the temperature is about 100 degrees). We never did fill it further because Willow wanted out about 15 minutes after I got in the water but it was enough water to make things more comfortable. In our test run of the pool, the hot water heater took about 1/2 hour to refill with hot water. Draining the pool took awhile (20 minutes?) with the small pump, but not a lot of effort. We hooked up the hoses and took turns sitting there pushing the button on the drill until the water was gone (it wasn't as gross as I expected, in case you are wondering. Didn't even get bloody until after the placenta delivered.). We just drained it into the rainwater drain outside the dining room sliding glass door. When there was only a teeny bit of water left that the pump couldn't pick up, we slashed the sides of the pool to let the air out and folded it enough to get it into a garbage bag.
I highly recommend doing a trial run a few weeks before delivery to work out any snags, figure out how long it takes to both empty and refill the hot water tank, etc. We discovered during our trial that the washing machine hookups were really tightly on (needed WD-40 to get the hose off); that it leaked some but it stopped after a few minutes so we just needed to put a bowl underneath; and that siphoning didnt work to empty the pool!
Other birth supplies: We got the kit our midwife recommended we order, with chux pads, bulb syringe, belly button clamp, gloves, sanitary pads and disposible undies, etc. We also had all the supplies in a list our midwife gave us (towels, big mixing bowl for the placenta, diapers, baby thermometer, etc.) around but didn't use much of it, things went so quick. And dont forget the camera and video camera (and dont be like us and discover that the video camera's batteries are dead just at the wrong moment).
Total cost was about $250 out of pocket for everything, and we really could have done without a lot of it.